White metal casting alloy and method of making same

ABSTRACT

A white metal casting alloy comprising as its essential ingredients tin, antimony, cadmium and lead in specified proportions. The invention further comprises a method of making said alloy comprising the steps of mixing a certain amount of the tin to be used with the lead and the antimony at a temperature of approximately 800* F. until the antimony is dissolved and then adding the remaining amount of the tin and mixing until the alloy becomes homogenous, and then cooling the mixture to approximately 475* F., at which point the cadmium is mixed and dissolved.

United States Patent 1 Darnell et a1.

1 1 WHITE METAL CASTING ALLOY AND METHOD OF MAKING SAME [75] Inventors:Robert S. Darnell, Washington, R.1.; Everett G. Morrissey, Bristol,Mass.

[73] Assignee: A. .1. Oster, C0., Providence, RI.

[22] Filed: Dec. 3, 1973 [21] Appl. No.1 420,948

[52] US. Cl 75/175 A [51] Int. Cl. C22c 13/00 [58] Field of Search75/175 A, 175 R [56] References Cited UNITED STATES PATENTS 2,097,72711/1937 Hanson et a1 75/175 A 2,180,139 11/1939 Dcitz 75/93 2,303,19311/1942 Bouton et a1. 75/166 2,891,511 6/1959 Fehling 120/424 [451 Mar.11, 1975 3,563,732 2/1971 Morisaki 75/175 A Primary Examiner-L DewayneRutledge Assistant ExaminerE. L. Weise Attorney, Agent, or Firm-Salter &Michaelson [57] ABSTRACT 3 Claims, No Drawings WHITE METAL CASTING ALLOYAND METHOD OF MAKING SAME BACKGROUND AND SUMMARY OF THE INVENTION Thepresent invention relates to casting alloys of the type frequently usedwith centrifugal rubber molds, such as during the casting ofjewelry andsimilar products. Alloys of this type embodying a high tin compositionare known in the trade as white metal alloys, and such alloys mustgenerally meet the following requirements. First of all, the cast alloymust lend itself to a high degree of polishing. Secondly, the cast alloymust i be capable of being effectively and efficiently plated. The castalloy must not be excessively brittle, since the cast articles,particularly where designed for jewelry use, frequently embody prongsthat must be bendable. In addition, it is desirable that the alloy becapable of being cast at as low a temperature as possible, since thelower the casting temperature, the longer the life of the molds,particularly where of rubber construction. In addition, the castingalloy must have good flowability.

White metal alloys, by definition, comprise a high tin content,generally upwards of 90% by weight. It is quite conventional to alloyantimony with tin, since the antimony adds a desired degree of hardnessto the alloy, without unduly or adversely affecting the castability ofthe tin. For example, one of the more conventional prior art alloys ofthis type is a product of American Smelting & Refining Co. and is calledOR-8 and comprises 92% tin and 8% antimony. Other prior art alloys. inaddition to tin and antimony, may also comprise lead in order toincrease the flowabilityof the alloy, or copper, in order to increasethe strength of the alloy. Examples of such prior art compositions arealloys comprising 94.5 parts tin, one part antimony, and 4.5 parts lead;or 92 parts tin, 4 parts antimony, and 4 parts lead; or 90.5 parts tin,7.5 parts antimony, and 2 parts copper.

Although prior art alloys of the type above mentioned have proven to begenerally satisfactory for their intended purposes, it has neverthelessbeen found that an alloy comprising as its essential ingredients tin,antimony, cadmium and lead, in proportions hereinafter to be specified,produces unexpectedly good results. As previously stated, the tin in thecomposition permits the alloy to be cast, while the antimony addsdesired hardness thereto. The presence of lead increases the flowabilityof the alloy and helps to prevent undesirable shrinking of the castalloy during cooling. The presence ofcadmium appears to make the alloymore ductile and bendable and at the same time lowers the temperature atwhich the alloy may be cast. More specifically, conventional prior artwhite metal alloys will normally be cast at a temperature of 650- 700 F.The alloy of the present invention, on the other hand, may be cast at atemperature of 500 550 F. This lower casting temperature is a pronouncedadvantage, since it promotes longer mold life, particularly where themolds are of rubber construction. In addition to the advantages juststated, the alloy of the present invention has all of the othernecessary characteristics of a good casting alloy, i.e., readily lendsitself to high polishing, is capable of being effectively plated, andhas good flowability. In fact, the present composition appears to bemore effective in these respects than the conventional prior art whitemetal casting alloys.

DESCRIPTION OF THE INVENTION In carrying out the present invention, ithas been found that an alloy comprising 90.5 92.5% tin, 2.9 3.1%antimony, 1.4 l.6% cadmium, and 3.3 3.771 lead achieves the advantageousresults hereinbefore discussed. Preferably, the alloy comprises 92% tin,3% antimony, 1.5% cadmium, and 3.5% lead. It is important to note thatsince cadmium adds toxicity to the alloy, an excessive amount (over 5%)would be detrimental. In lesser amounts, however, i.e., approximatelyl.5% of the alloy, the presence of the cadmium is not only notdetrimental, but appears to be highly desirable in that it adds to theductility of the alloy and at the same time lowers the castingtemperature thereof. In addition, it has been found by chemical analysisthat the alloy of the present invention has a unique crystallinestructure of its own, which can be described as true crystalline, incomparison with the amorphous structures of OR-8, which indicate atwo-phase eutectic alloy. This is thought to be attributable to thepresence of cadmium in the composition.

In making the present composition, the lead and antimony are mixed withofthe tin to be used at a temperature of approximately 800 F. until theantimony dissolves, at which point the other 25% of the tin is added tothe mixture and is homogeneously dispersed therein, as by conventionalmixing apparatus. After the mixture has become homogenous, it is allowedto cool until it reaches a temperature of 475 F. or slightly lower, atwhich point the cadmium is added and mixed therein until dissolved. Thealloy is now ready for easting and, as previously stated, may be castand formed at temperatures of 500 550 F.

Also of importance is the purity of the metals used. Thus, the tinshould be 99.89l% pure, while the cadmium should be 99.99% pure and thelead 99.9% pure in order to obtain the most desirable results.

In addition to the hereinbefore discussed advantages of the presentalloy, it has additionally been found that it is possible to plate goldby immersion onto the alloy. More specifically, the system used was agold plating solution known commercially as Tivaklad GI-3 sold by TivianLaboratories, Inc., of Providence, Rhode Island. It consists of abuffered gold chelate having the following composition:

Gold 1 troy ounce Chelate 2 pounds Water 1 gallon pH 3.0 Temperaturel40-l50 F.

The capability of the present alloy to be gold plated by immersion is acompletely unexpected result and constitutes a further advantage of thepresent alloy, since gold plating by immersion has not heretofore beenpossible with conventional prior white metal casting alloys.

As this invention may be embodied in several forms without departingfrom the spirit or essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within themetes and bounds of the claims or that 3. The method of making the alloyof claim 1, comprising the steps of mixing together at approximately 800F. the lead, the antimony and approximately of the tin until theantimony has dissolved. then adding the remainder of the tin to themixture and mixing same until the mixture has become substantiallyhomogenous, then causing the mixture to cool to 475 F. or slightly less,then adding the cadmium and mixing until the latter has dissolved.

1. A WHITE METAL ALLOY CONSISTING OF 91.5 - 92.5% TIN, 2.9 3.1%ANTIMONY, 1.4 - 1.6% CADMIUM, AND 3.3 - 3.7% LEAD.
 1. A white metalalloy consisting of 91.5 - 92.5% tin, 2.9 -3.1% antimony, 1.4 - 1.6%cadmium, and 3.3 - 3.7% lead.
 2. The alloy of claim 1 furthercharacterized in that said tin is at least 99.891% pure, said cadmium isat least 99.99% pure, and said lead is at least 99.9% pure.